The purpose of the study is to examine the effects of different exercise intensity training programs upon blood vessel function and circulating blood cells involved in blood vessel repair in young women. The long term effects of exercise may be beneficial to cardiovascular health and it is important to understand the training methods that are the most beneficial. In particular we aim to determine if brief maximal exercise improves the function and stiffness of blood vessels and enhance blood vessel repair. Two methods of exercising are being compared, exercise bouts involving intermittent exercise and exercise completed all at once, but at a very high intensity.